Temperature dependence of morphology and diameter
of silicon nanowires synthesized by laser ablation
Y.Q. Chen, K. Zhang, B. Miao, B. Wang, J.G. Hou
*
Structure Research Laboratory, Center for Physical Sciences, University of Science and Technology of China, Hefei 230026, China
Received 15 March 2002
Abstract
The silicon nanowires (SiNWs) with different diameters and morphologies were synthesized by laser ablation of a
target containing metals over a temperature range 910–1120 °C. The octopus-shaped wires of larger diameters were
formed in lower temperature zone (910–960 °C), while SiNWs and silicon nanoparticle chains of smaller diameters in
higher temperature zone (960–1120 °C). The distribution of the morphology and diameter of SiNWs as a function of
growth temperature differs from that reported by thermal evaporation of SiO powders. The study shows that the
morphology and diameter of SiNWs synthesized by laser ablation not only correlate closely with the growth
temperature of SiNWs, but also with the nature of a catalyst. By change of nucleation temperature and critical
nucleus size of nucleus droplets in vapor–liquid–solid (VLS) growth process, a catalyst can change relationships
between the morphology, diameter, and growth temperature of SiNWs. Ó 2002 Elsevier Science B.V. All rights
reserved.
1. Introduction
One-dimensional nanostructured materials,
such as nanotubes [1,2], nanowires [3–8], and
nanobelts [9], are a burgeoning and intriguing re-
search area both for their fundamental scientific
issues in meso-physics phenomena and for poten-
tial nano-device application. Silicon is one of the
most important electronic materials and holds
considerable technological promise for device ap-
plications. Therefore much attention has been paid
recently to the investigation of SiNWs. Until now,
a variety of techniques on synthesizing SiNWs
have been developed, including lithography and
etching techniques [10,11], scanning tunneling mi-

not only correlate closely with the growth tem-
perature of SiNWs, but also with the nature of a
catalyst.
2. Experimental
The experimental apparatus used for the pre-
sent work is similar to the one described previously
[3,17]. An alumina tube was mounted inside a
horizontal tube furnace. A target was made by
compressing Si powders (purity 99.99%) with 5
mol% Zr (purity P 92%; impurities, Mg, Fe, Ge,
Ca, Cl). The target was placed at the center inside
the furnace. A strip-like Si substrate (68 mm in
length and 20 mm in width) was placed at the
outlet end, near a cooling copper finger for col-
lecting the deposited products. There existed a
temperature gradient from center to the gas outlet
end of the furnace. A PtRh–Pt thermocouple was
used to measure the temperature distribution in
the alumina tube. After the tube had been evacu-
ated to 0.01 Torr by a mechanical vacuum pump,
5% H
2
–Ar gas mixture, as a carrier gas, was in-
troduced and kept flowing at a flow rate of 50
sccm. The pressure in the tube was controlled at
300 Torr. Then the furnace was heated to 1200 °C
at the central region. After the temperature and
pressure in the tube had been stabilized, pulsed
laser beam from an Nd:YAG laser (wavelength
532 nm, pulse width 7–8 nm, frequency 10 Hz,

analysis indicated that the nanoparticles at the tips
of the nanowires only contained silicon and oxy-
gen. Within the detection limit of EDS measure-
ments (% 0:5%), no evidence of existence of Zr or
any other elements was detectable on the tip. Even
though the metal silicide was not found, it is be-
lieved that the function of these Si tips of the
nanowires is analogous to that of the metal silicide
catalyst in VLS growth process [15,16]. Namely,
melt Si nanoparticles may act as a nucleus for the
nanowires. In zone B (960–1080 °C), a large
quality of what is called silicon nanoparticle chains
were formed (Figs. 1b,c). Clearly, the diameter of
the silicon nanoparticle chain decreases with re-
ducing the growth temperature. Lee et al. [16] have
intensively studied the growth mechanism of the
silicon nanoparticle chains. They proposed that
nucleation and growth occurring alternatively re-
sulted in the formation of chains of silicon nano-
particles. The formation of the kinks of silicon
nanoparticle chains resulted from a change of
growth direction of the SiNWs. We examined the
kinks by using energy dispersive spectroscopy and
Y.Q. Chen et al. / Chemical Physics Letters 358 (2002) 396–400 397
confirmed that the kinks only contained silicon
and oxygen.
It should be noted that, from zone A to zone
B, the temperature dependence of the morphol-
ogy and diameter of the SiNWs is similar to that
by thermal evaporation, reported recently by

zone C, a relatively lower temperature zone (Fig.
1d). It is interesting that the octopus-shaped sili-
con nanowires synthesized by thermal evaporation
of SiO powders were found at higher growth
temperature of more than 1230 °C [15]. Focusing
on the octopus-shaped structure, it can be seen
that two or more branches share the same tip,
which suggests that the tip might act as the nu-
cleation site for two or more branches when the
diameter of tip was large enough. Moreover, a
bifurcation phenomenon of the silicon nanowires
was also observed, which may be attributed to
renucleation of the crystal silicon in growth pro-
cess. On the other hand, the diameter of each
branch of the octopus-shaped nanowires decreases
gradually as the distance from the tip increases.
TEM investigations revealed that there existed two
kinds of structures in the entire length of the
branch. One was a crystal silicon core sheathed
with a thick amorphous outer layer of silicon ox-
ide, which originated from the tip and terminated
at reaching a certain length. The other, the rest of
the branch, was a complete amorphous silicon
oxide. Fig. 3 is the HRTEM image of the interface
(arrow in Fig. 1d) of two kinds of structures along
the axis of the branch. It can be seen that the
growth direction of the crystal silicon was h 111i,
which is consistent with the fact that the growth
direction of SiNWs synthesized by metal catalyzed
VLS growth is predominantly h111i [3,14]. The

of the tips of larger diameter in zone C. This then
explains why these octopus-shaped wires could be
deposited in zone C, a relatively lower temperature
zone. Furthermore, it is known that the diameters
of the nanowires formed in VLS process have re-
lation to the critical diameter dc of the liquid
droplets nucleated in VLS process. Droplets larger
than d
c
will become stable nuclei, whereas droplets
smaller than d
c
will disappear gradually. The crit-
ical nucleus size can be expressed as r
Ã
¼
ðÀ2cÞ=DF
v
; where c is the specific interfacial free
energy of the condensate–vapor interface and DF
v
is the bulk free energy change per unit volume
[21,22]. We assume that the addition of Mg and Ge
into Si nanoparticles would increase interfacial
free energy c. Therefore r
Ã
becomes larger, which
means that the droplets with larger diameters
(> r
Ã

(2000) 1116.
[5] H.Y. Peng, X.T. Zhou, N. Wang, Y.F. Zheng, L.S. Liao,
W.S. Shi, C.S. Lee, S.T. Lee, Chem. Phys. Lett. 327 (2000)
263.
[6] W.S. Shi, Y.F. Zheng, N. Wang, C.S. Lee, S.T. Lee, Adv.
Mater. 13 (2000) 591.
[7] M.H. Huang, Y. Wu, H. Feick, N. Tran, E. Weber, P. Yang,
Adv. Mater. 13 (2001) 113.
[8] Y. Wu, B. Messer, P. Yang, Adv. Mater. 13 (2001) 1487.
[9] Z.W. Pan, Z.R. Dai, Z.L. Wang, Science 291 (2001) 1947.
[10] H.I. Liu, N.I. Maluf, R.F.W. Pease, J. Vac. Sci. Technol. B
10 (1992) 2846.
[11] H. Namatsu, S. Horiguchi, M. Nagase, K. Kurihara,
J. Vac. Sci. Technol. B 15 (1997) 1688.
[12] T. Ono, H. Saitoh, M. Esashi, Appl. Phys. Lett. 70 (1997)
1852.
[13] D.P. Yu, Z.G. Bai, Y. Ding, Q.L. Hang, H.Z. Zhang,
J.J. Wang, Y.H. Zou, W. Qian, G.C. Xiong, H.T. Zhou,
S.Q. Feng, Appl. Phys. Lett. 72 (1998) 3458.
[14] H.Y. Peng, Z.W. Pan, L. Xu, X.H. Fan, N. Wang,
C.S. Lee, S.T. Lee, Adv. Mater. 13 (2001) 317.
[15] Z.W. Pan, Z.R. Dai, L. Xu, S.T. Lee, Z.L. Wang, J. Phys.
Chem. B. 105 (2001) 2507.
[16] S.T. Lee, N. Wang, Y.F. Zhang, Y.H. Tang, MRS Bull.
August (1999) 36.
[17] T. Guo, P. Nikolaev, A. Thess, D.T. Colbert, R.E. Smalley,
Chem. Phys. Lett. 243 (1995) 49.
[18] R.S. Wagner, W.C. Ellis, Appl. Phys. Lett. 4 (1964) 89.
[19] G.V. Raynor, The Physical Metallurgy of Magnesium and
its Alloys, Pergamon, London, 1959.